1. Field of the Invention
The present invention relates to imaging sub-surface structures, particularly hydrocarbon reservoirs and fluids therein, and more particularly to cross-well and borehole-to-surface electromagnetic (EM) surveying.
2. Description of the Related Art
Cross-well and borehole-to-surface electromagnetic (EM) surveying have involved continuous-wave (CW) EM sources placed in one borehole and receivers/sensors which detected the phase and amplitude of the EM signal in a distal borehole, using multiple source and receiver positions. The data readings were used to generate a synthetic time domain version of the signal, and inferred transit times were used along with source/receiver geometry to create a 2D conductivity matrix or image of the inter-well plane via inversion with ray-tracing.
Brine which is electrically conductive is everpresent in hydrocarbon reservoirs, and the presence of brine attenuated EM signals in proportion to their frequency. The presence of brine, as well as the large inter-well distances on the order of 1 kilometer or more, and thermal noise limits in conventional receivers caused continuous-wave EM surveys to require very low frequency operation, usually about 200 Hz. The low frequency operating range which was required severely limited cross-well imaging resolution as it: a) is in a diffusive regime, and b) had a very large wavelength. At present, so far as is known, a spatial resolution of only 1/10th to 1/20th of the inter-well spacing has been obtainable.
Since practical spacing for boreholes in hydrocarbon reservoirs usually span hundreds to thousands of meters and such reservoirs are usually associated with electrically conductive brines, significant EM signal attenuation across a reservoir has been encountered. Such attenuation is frequency-dependent, such that higher frequencies are attenuated more than lower frequencies. Since higher frequencies have shorter wavelengths, and therefore afford better imaging resolution, it would be advantageous to operate at the highest frequency that still gives a detectable signal after transiting the reservoir region of interest. However, the presence, concentration and distribution of brines are generally unknown prior to investigation, and the optimal frequency for investigating the reservoir with EM surveying could not be determined in advance.